This invention is related to highly localized infrared (IR) spectra and chemical mapping on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength pulsed IR source illuminating the sample in the region of the AFM probe tip and producing a measurable wavelength dependent radiation-sample interaction, and in particular using an electric field enhancing probe tip to produce tip-sample interaction at lower power levels leading to improved spatial resolution.
IR spectroscopy is a useful tool in many analytical fields such as polymer science and biology. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of surfaces, such as location of different materials or molecular structures. Recently, a technique based on use of an AFM in a unique fashion to produce such localized spectra has been developed. This work was described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” Optics Letters, Vo. 30, No. 18, Sep. 5, 2005 and related publications by Dazzi et al. In addition, recent improvements have been described by Lu and Belkin as described in F. Lu, and M. A. Belkin, “Infrared absorption nano-spectroscopy using sample photoexpansion induced by tunable quantum cascade lasers,” Optics Express 19, 19942 (2011), which is incorporated by reference. Those skilled in the art will comprehend the details of the technique in the publications but the technique will be described briefly herein for clarity.
AFM's are known in the art. The important aspects of an AFM for the current invention are shown in FIG. 1. A light beam 1 is directed at an angle to a cantilever arm 2 which reflects the beam to a photo-detector 4. Typically the photodetector is a 4-quadrant type, and when the cantilever is placed in the AFM the lever is positioned such that the reflected beam is centered on the photo-detector. The cantilever typically has a probe tip that interacts with sample 3. The interaction can be contact, intermittent contact, non-contact including attractive and/or repulsive forces. Deflection of the cantilever vertically due to contact with the sample causes the beam spot to move on the detector, generating a difference signal from the detector quadrants. This type of AFM setup is called an optical lever arm, and commercially available AFM's using this technique can measure deflections of the lever on a sub-angstrom scale. There are other options to detect the deflection of the cantilever which can also be used, such as optical interferometry. Other deflection detectors may not generate as much information from the sample as the beam deflection detection. Not shown, but typically present in an AFM, is a scanner to generate relative movement between the tip and sample, and feedback electronics which typically servo the sample or tip up and down in response to height variations of the sample to maintain a desired interaction between the tip and sample. This vertical servo signal vs lateral position creates a topographical map of the surface which in, commercial AFM's, can attain atomic resolution. A wide variety of variations of the AFM exist with different types of probes and so on for measurements other than topography. For instance, in a co-pending application by some of the inventors of this application, a version of an AFM configured to measure thermal properties of a surface is described.
The AFM set-up used for the published work on IR spectroscopy is shown schematically in FIG. 2. In this set-up, the sample 3 is mounted on a ZnSe prism 8, or prism made from other suitable materials, which primarily does not absorb the radiation of interest. A modulated IR source 9 is directed into the prism. In some embodiments the prism is made at an angle such that the beam is in Total Internal Reflection in order for the beam to be propagative in the sample and evanescent in the air. Thus only the sample is significantly exposed to the laser radiation, and the AFM probe 2 is minimally exposed to beam 9. The IR source that is both variable in wavelength and has a pulsed output. The IR source may be pulsed asynchronously or as in the Lu and Belkin paper and copending application Ser. No. 13/236,615, the source may be pulsed at a frequency corresponding to a resonance of the cantilever. The probe 2 is positioned by scanner 6 to interact with a region of the sample. The probe may be held at a desired level of interaction (for example a desired force, cantilever deflection, oscillation amplitude, resonant frequency shift for example) by feedback electronics 5. Feedback electronics can be analog, digital, digital computation any combination of the above. Both the vertical and lateral deflection signal directly from the photo-detector 4, as well as the feedback signal, can be monitored at 7.
Referring to FIG. 3, when the IR source is pulsed, the sample 3 may absorb some of the energy, resulting in a fast thermal expansion of the sample as shown in the Figure. This has the effect of a quick shock to the cantilever arm 2, which if the ability of the cantilever to respond to this shock is slower than the shock will result in exciting a resonant oscillation in the cantilever arm. In the case of asynchronous pulses with a repetition rate in the kHz range, the shock will result in a decaying oscillation or “ringdown,” as shown in the figure. In the case of pulses that are synchronized with a resonance of the cantilever or that have a repetition period shorter than the ringdown decay time, a continuous cantilever oscillation can be generated. Because the IR energy is ideally contained within the sample, this shock is due primarily to rapid sample expansion as minimal IR energy is absorbed by the cantilever itself. In the case of asynchronous IR pulses, transient cantilever response signal is often too fast for the feedback electronics, but can be observed directly from the photodetector. Thus the cantilever rings in the manner shown in FIG. 3 while still in contact with the surface, an effect called “contact resonance”. The absolute deflection, amplitude and frequency characteristics of the contact resonance vary with the amount of absorption as well as other properties, such as the local hardness, of the localized area around the probe tip. Also, depending on the direction of the expansion, vertical resonances, lateral resonances or both can be excited. If the tip is to the side of the absorbent material, this will typically cause a stronger lateral response in the cantilever. By repeating the above process at varying wavelengths of the IR source, an absorption spectra on a very localized scale is achieved. By scanning the probe to various points on the sample surface and repeating the spectra measurement, a map of IR spectral surface characteristics can be made. Alternatively, the wavelength of the IR source can be fixed at a wavelength that is characteristic of absorption of one of the components of the sample. The probe can then be scanned across the sample surface and a map of the location of that component can be generated.
The technique is often referred to as PTIR (Photo Thermal Induced Resonance) spectroscopy. A variety of improvements to the technique are disclosed in the parent application to the current application as well as other family members U.S. application Ser. Nos. 12/315,859 and 13/236,666 which are incorporated in their entirety by reference. Among those improvements includes the disclosure of suitable alternative bench top radiation sources to the previously used IR source (a free electron laser) as well as a variety of techniques to improve signal strength and spatial resolution for a variety of samples and applications. For some sample geometries and measurements, more improvement is desirable. It is the object of this invention to provide further enhancement of the PTIR technique for certain sample and measurement scenarios.